Effect of End-Chain Polarity on the Mesophase Stability of Some Substituted Schiff-Bases

Abstract

The mesophase transition temperatures and heats of transitions of two series of novel substituted benzylideneaminoacetophenones were measured. The geometry of corresponding members of the two series was kept closely similar while changing their polar character by selecting n-alkoxy groups as substituents for series A and by replacing a methylene group by an oxygen atom in series B. Oxygen substitution reduces nematic stability relative to the smectic phase, and lowers the transition temperatures of the mesophases throughout the series. The heats of transitions and the associated entropy changes are generally higher in series B. The thermal properties are insensitive to the location of the oxygen atom in the end-chain. The data indicate that dispersion forces dominate over permanent dipole interactions in determining mesophase stability.